The scanning electron microscope (SEM) scans a sample with an electron beam in a two-dimensional manner, detects a secondary electron emitted from the sample, and creates an image using the detected electron. The SEM has a similar configuration to an optical microscope that includes an object lens and creates an image using light reflected from the surface of a sample. An electron gun corresponding to a light source of the optical microscope serves to generate an electron and accelerate the electron. The electron gun supplies a group of electrons, which is used in the form of an electron beam.
Since an electron in an atom has constant energy at a specific energy orbit through an electric force acting between the electron and the nucleus of the atom, the electron hardly escapes from the atom as long as energy is not applied from outside. However, when energy equal to or more than an energy barrier (work function) is applied, the electron escapes to the outside. That is, when a metal used as a filament of an electron gun, such as tungsten, is heated at high temperature, electrons bound to the atom escape from the nucleus to the vacuum.
The electron gun for an electron microscope may be divided into a thermionic electron gun and a field emission electron gun, depending on their electron emission methods. A filament used as a cathode in the thermionic electron gun is usually formed of tungsten wire, bent in a hair pin shape having a V-shaped end, and has a diameter of approximately 100 μm. Since tungsten has a low work function value of 4.5 eV and a high melting point of 3,659K, tungsten is used as the filament in many cases. The thermionic electron gun directly applies a current to the filament until the temperature rises to about 2,200K. In a high-quality electron microscope, a filament formed of LaB6 (lanthanum hexaboride) is used to increase electron density, and heated to a temperature of 1,800K. When atoms are adsorbed to the surface of LaB6, electron emission performance is significantly degraded. Thus, a high vacuum must be maintained in the electron microscope.
The thermionic electron gun includes a filament at the top thereof and an anode plate at the bottom thereof. The filament is surrounded by a Wehnelt cylinder, and the anode plate serves as an acceleration electrode. In the Wehnelt cylinder, a voltage having a smaller negative value than the filament used as a cathode is formed as a bias voltage. Thus, electrons emitted from the filament are focused in the center by a repulsive force. The thermionic electron gun accelerates the electrons through a voltage difference applied between the anode plate and the filament serving as the cathode, and emits the accelerated electrons emitted from the filament downward, thereby forming an electron beam.
The field emission electron gun includes a tip serving as a cathode and primary and secondary anodes. The tip is made of tungsten, and manufactured in a sharp shape to have a curvature radius of approximately 600 to 2,000 A. Thus, when a strong electric field is applied, the tip reduces the thickness of a potential barrier such that electrons can easily escape from the surface of tungsten through the tunneling effects. Since the field emission electron gun acquires an electron beam with uniform energy from the tip, the field emission electron gun can form ultra-high electron beam brightness and a small intersection, thereby acquiring high resolution. The primary anode applies a high voltage of several kV such that electrons are emitted from the tip, and the secondary anode accelerates the electrons. At this time, an acceleration voltage of several tens of kV is applied between the secondary anode and the tip. The field emission electron gun may be divided into a cold cathode field emitter (CFE), a thermally assisted field emitter (TFE) and a Schottky field emitter (SE).
The center axis of an electron beam emitted from a filament block including the filament and the Wehnelt cylinder in the thermionic electron gun or an electron beam emission unit including the tip and the primary anode in the field emission electron gun must be aligned with the center axes of a focusing lens and the anode plate of the thermionic electron gun or the secondary anode of the field emission electron gun. When the center axes are misaligned from each other, a problem may occur in electron beam alignment, and the number of electrons reaching a sample may decrease. In this case, an aberration may occur in an electromagnetic field lens, and the sample examination resolution may be degraded. Furthermore, in the related art, a metal gasket has been used for vacuum sealing, which means that a new metal gasket must be used during a disassembling/assembling operation. Thus, the operation takes a lot of cost, and a metal bellows requires a high manufacturing cost. In order to perform sealing using a metal gasket, an operator must apply a predetermined force, and may have inconvenience in the disassembling/assembling operation because a number of assembling screws are need.
U.S. Pat. No. 4,663,525 discloses a method for electron gun alignment in electron microscopes. In this method, however, when a tip position is mechanically adjusted, a bellows is inserted into an electron gun in order to maintain a filament connection portion in a vacuum state. Thus, the structure of the electron gun becomes complex.
The electron gun must maintain a vacuum state therein such that a flow of electron beam does not collide with internal gas. The thermionic electron gun requires a high vacuum of 10−3 Pa to 10−5 Pa, and the field emission electron gun emitting a high-density (high brightness) electron beam required for a high-resolution electron microscope is operated at a vacuum of approximately 10−6 Pa or less, which is much higher than the thermionic electron gun. In particular, the cold field emission electron gun among the field emission electron guns requires a vacuum of 10−7 Pa or less. Therefore, a container for maintaining an ultra high vacuum of the field emission electron guns for emitting a high-density electron beam is usually formed of stainless steel (STS) suitable for maintaining a high vacuum, because STS has a relatively low outgassing rate.
In order to prevent a disturbance of electron flow, a stray magnetic field must be blocked while a high vacuum is maintained, the stray magnetic field permeating from the outside of the electron gun. This is because, since electrons are charged particles and have a small mass, the electrons are sensitive to a magnetic field. In order to shield a magnetic field, a material with high permeability needs to be used, but STS has a small relative permeability (μr) of 1 to 10. Therefore, in the related art, an electron beam emission source is surrounded by Mu-metal with a relative permeability of 20,000 to 50,000 or permalloy with a relative permeability of 8,000, in order to shield a magnetic field. However, the permalloy or Mu-metal is expensive and requires a high processing cost. Thus, when a material having a low outgassing rate and high relative permeability is used to maintain a high vacuum, economical efficiency and manufacturing facilitation can be secured.
Korean Registration Patent 10-0473691 discloses an aluminum (Al or Al alloy)-based vacuum chamber member. However, this patent relates to a material which exhibits a high corrosion resistance to corrosive gas or plasma. Thus, the material is very different from a material for forming a vacuum chamber which is capable of preventing a stray magnetic field while maintaining a high vacuum.